UX CSS Layout

This skill covers CSS file organization, naming, standard sizes, programmatic layout (SplitView, Grid, scrollable), responsive patterns, and text overflow handling.

1. File Organization

CSS files are co-located with their TypeScript components:

src/vs/base/browser/ui/button/
    button.ts
    button.css
src/vs/workbench/contrib/myFeature/browser/
    myFeature.ts
    media/
        myFeature.css

Import CSS directly in the TS file:

import './media/myFeature.css';
// or for base widgets:
import './button.css';

Workbench-level global styles live in src/vs/workbench/browser/media/.

2. Class Naming

• monaco- prefix for all major components: .monaco-workbench, .monaco-split-view2, .monaco-scrollable-element
• Modifier classes: .monaco-split-view2.vertical, .monaco-split-view2.horizontal
• State classes: .visible, .focused, .active, .highlight
• Feature-specific classes use kebab-case without prefix: .my-feature, .outline-pane, .welcome-view-content

3. Standard Sizes

For padding/margin/gap, border-radius, font-size/font-weight, codicon size and border width, prefer the design-system size tokens over raw px — see §10 Design-System Size Tokens. Canonical reference: .github/instructions/design-tokens.instructions.md (auto-injected for src/vs/**/*.css).

4. CSS Selector Quality

Never Add New !important

Do not introduce !important in new or modified CSS. When a declaration loses the cascade, inspect the competing selector and increase specificity with the smallest appropriate component, workbench, or state-class prefix instead. Existing !important declarations may be preserved and must not be removed mechanically during unrelated edits. Do not copy them, add new ones, or use them to avoid understanding selector ownership.

The narrow exception is shared focus/active-outline suppression, where outline: 0 !important is intentionally used to override native or global focus indicators and prevent flashing outlines during pointer activation. Keep this exception scoped to focus-indicator behavior; feature styling must still resolve cascade conflicts through selector specificity.

5. SplitView Layout

File: src/vs/base/browser/ui/splitview/splitview.ts

For splitting views with draggable sashes (either horizontal or vertical):

const splitView = new SplitView(container, {
    orientation: Orientation.VERTICAL,
    proportionalLayout: true,
    styles: { separatorBorder: asCssVariable(sashBorder) }
});

// Each view implements IView:
const myView: IView = {
    element: myDomNode,
    minimumSize: 100,
    maximumSize: Number.POSITIVE_INFINITY,
    onDidChange: Event.None,
    layout(size, offset) { /* resize content */ }
};

splitView.addView(myView, Sizing.Distribute);

Use LayoutPriority.High / .Low to control which views resize first when space is constrained. Use snap: true to allow views to snap closed.

6. Grid Layout

File: src/vs/base/browser/ui/grid/grid.ts

For 2D layouts (used by editor groups):

const grid = new Grid(initialView);
grid.addView(newView, Sizing.Distribute, referenceView, Direction.Right);

7. Scrollable Content

Three classes for different needs:

const scrollable = new DomScrollableElement(contentNode, {
    horizontal: ScrollbarVisibility.Auto,
    vertical: ScrollbarVisibility.Auto
});
this._register(scrollable);
container.appendChild(scrollable.getDomNode());
scrollable.scanDomNode(); // call after content changes

8. Responsive Layout

VS Code does not use CSS media queries. Instead, it uses a programmatic constraint-based layout system:

• IView.minimumSize / maximumSize — views declare their size constraints.
• SplitView and Grid distribute space according to constraints and LayoutPriority.
• ResizeObserver is used for container-aware sizing (e.g., editor auto-layout).
• The window is treated as a fixed viewport; space is distributed via sash-based resizing.

When building a responsive component:

1. Set minimumSize / maximumSize appropriately.
2. Use LayoutPriority.Low for panels that should collapse first.
3. Use snap: true for panels that should snap closed when too small.
4. Fire onDidChange when your constraints change dynamically.

9. Text Overflow & Ellipsis

All text labels that can be truncated by a resizable container must use the ellipsis pattern. Clipped text without an ellipsis is a visual bug.

Standard Ellipsis Pattern (CSS)

The three-property combo is required — all three must be present:

.my-label {
	overflow: hidden;
	white-space: nowrap;
	text-overflow: ellipsis;
}

Common Locations That Need Ellipsis

Flex Container Gotchas

Flex children default to min-width: auto, which prevents text-overflow: ellipsis from working because the flex item refuses to shrink below its content width. Fix this by setting min-width: 0 on the flex child:

/* WRONG — ellipsis will NOT trigger inside a flex container */
.flex-parent {
	display: flex;
}
.flex-parent > .label {
	overflow: hidden;
	text-overflow: ellipsis;
	white-space: nowrap;
}

/* CORRECT — add min-width: 0 so the flex item can shrink */
.flex-parent > .label {
	min-width: 0;          /* ← this is the fix */
	overflow: hidden;
	text-overflow: ellipsis;
	white-space: nowrap;
}

This pattern is used throughout VS Code — for example, .monaco-icon-label-container sets min-width: 0 and flex: 1 to allow label text to truncate.

Fixed vs Flexible Elements

When a row has both fixed-size elements (icons, action buttons) and flexible text:

.row {
	display: flex;
	align-items: center;
}
.row > .icon {
	flex-shrink: 0;        /* icon never shrinks */
	width: 16px;
}
.row > .label {
	flex: 1;               /* label takes remaining space */
	min-width: 0;          /* allows shrinking below content width */
	overflow: hidden;
	text-overflow: ellipsis;
	white-space: nowrap;
}
.row > .actions {
	flex-shrink: 0;        /* action buttons never shrink */
}

This is the standard pattern for tree rows, list items, tab labels, and view pane headers.

Hover for Full Text

When text is truncated with ellipsis, the full text must be accessible via hover tooltip. Use IHoverService.setupDelayedHover() with the full untruncated text so users can read it:

this._register(this.hoverService.setupDelayedHover(labelElement, {
	content: fullLabelText,
}));

For IconLabel and list/tree renderers, this is handled automatically. For custom DOM, you must add it manually.

Anti-Patterns (NEVER DO)

• Never let text clip without an ellipsis — if overflow: hidden is set, text-overflow: ellipsis must also be set.
• Never rely on a fixed pixel width for text that could be localized — localized strings vary in length.
• Never use text-overflow: ellipsis without overflow: hidden and white-space: nowrap — all three are required.
• Never forget min-width: 0 on flex children that need to truncate.
• Never truncate text without providing a hover/tooltip for the full string.

10. Design-System Size Tokens (spacing, radius, font, codicon, stroke)

VS Code ships a design-system size ramp, registered in src/vs/platform/theme/common/sizes/baseSizes.ts and emitted as --vscode-* CSS variables. When writing or editing CSS, prefer the token var over a raw px value wherever a token exists. The full tables + rationale live in the auto-injected .github/instructions/design-tokens.instructions.md (canonical source — keep this section in sync with it). This section captures the decision logic for deeper styling tasks.

Every --vscode-* size var you reference must already exist in build/lib/stylelint/vscode-known-variables.json ("sizes" array, alphabetically sorted) or stylelint/hygiene fails. Adding a new token means adding it both in baseSizes.ts and that JSON file.

Spacing — padding, margin, gap

Scale (px): 0, 2, 4, 6, 8, 10, 12, 16, 20, 24, 28, 32, 36, 40 → --vscode-spacing-sizeNone, --vscode-spacing-size20 … --vscode-spacing-size400 (token number = px × 10, so size200 = 20px).

What matters is the value, not the token. Adopting the var() is optional — a raw px value is fine as long as it lands on the scale. What breaks rhythm is an off-scale value (3, 5, 7, 14, 26px…). Snap off-scale values to the nearest scale value, ties round up (5px → 6px, 3px → 4px, 1px → 2px, 26px → 28px). Each length of a shorthand is checked independently (0 5px → 0 6px). Leave auto, %, em/rem, var()/calc() untouched.

Corner radius — border-radius

Snap map for off-scale literals (ties round up): 2→xSmall, 3,4→small, 5,6→medium, 7,8→large, 10,11,12→xLarge, 14,16,18,20→xLarge, 999→circle.

• Pills (radius ≈ half the element height — e.g. 28h/14r, 36h/18r, 22×22/11r) → --vscode-cornerRadius-circle, not xLarge. The literal-nearest token would square them and lose the fully-rounded intent.
• Leave untouched: 50%, 0, 0px, inherit, any calc()/var(). Preserve !important.

Font size & weight

Generic UI ramp — pair a size token with a weight token ("Strong" = matching size token + semiBold, never a separate size):

Generic weights: --vscode-fontWeight-regular (400), --vscode-fontWeight-semiBold (600).

Deprecated — --vscode-bodyFontSize (13) → --vscode-fontSize-body1, --vscode-bodyFontSize-small (12) → --vscode-fontSize-label1, --vscode-bodyFontSize-xSmall (11) → --vscode-fontSize-body2.

The legacy Agents-specific --vscode-agents-fontSize-* and --vscode-agents-fontWeight-* tokens are also deprecated; use the matching generic tokens.

• No medium (500). font-weight: 500 is off the ramp — snap to semiBold. Likewise 700/bold → round to the nearer of 400/600.
• "Strong" is not a separate size. "Body 1 Strong" = the matching --vscode-fontSize-* size token + semiBold. Never add a strong size.
• normal ≡ 400 → regular. Leave inherit, lighter, bolder, var()/calc() untouched.

Codicon size — icon font-size

Codicons are only ever 16px or 12px — never 14px or any in-between value.

Compact-glyph convention: when sizing an icon at the compact 12px size, also swap the registered glyph to its *Compact variant (e.g. Codicon.close → Codicon.closeCompact, Codicon.add → Codicon.addCompact). CSS font-size alone only scales the icon — it does not change to the visually-optimized compact glyph; that requires changing the registered icon (Action2 icon: / renderIcon). Only swap the glyph when no CSS selector targets the original glyph class (e.g. .codicon-close); selectors keyed on the glyph class (.codicon-add, .codicon-chevron-down) break when the class becomes -compact, so update those selectors too (or size via a glyph-independent wrapper class like .monaco-button). Some icons (settings/sliders, agent, vm, info, lock, plus) have no compact variant — keep the regular glyph at 12px.

Stroke — border width

A single stroke thickness: 1px → --vscode-strokeThickness. Applies to the border: 1px solid <color> shorthand and border-width: 1px. Other widths have no token — leave them.

/* prefer */  border: var(--vscode-strokeThickness) solid var(--vscode-widget-border);
/* avoid  */  border: 1px solid var(--vscode-widget-border);

Key Files